1. Field of the Invention
The present invention relates to an optical transmission line including positive and negative dispersion fibers, and repair of such a transmission line.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a typical configuration of a wavelength division multiplexing optical amplification relay transmission system.
Referring now to FIG. 1, the WDM transmission optical system is typically made up of, for example, an optical send station (OS) 1, an optical receive station (OR) 2, optical fiber transmission lines 3 for connecting the sending and receiving stations, and several optical amplifiers 4 set up at the required intervals along the optical fiber transmission lines 3.
The optical send station 1 includes several optical transmitters (E/O) 1A for outputting many optical signals of differing wavelengths, a wavelength-multiplexer device 1B for multiplexing the many optical signals into a WDM signal light, and a post amp 1C for amplifying the WDM signal light from the wavelength multiplexer device 1B to the required level and outputting the WDM signal light to the optical transmission lines 3.
The optical fiber transmission lines 3 contain many relay divisions for connecting to optical send stations 1 and optical receive stations 2.
The optical amplifier 4, mounted in the transmission line, optically amplifies the WDM signal light and sends it to the optical receive station 2.
The optical receive station 2 includes a pre-amp 2C for amplifying to the WDM signal light of each of the wavelength bands transmitted by the optical fiber transmission line 3 to a required level, a wavelength demultiplexer 2B for dividing up the optical output from the pre-amp 2C into many optical signals according to the wavelength, and several optical receive stations (O/E) 2A for processing the many optical signals as they are received.
FIG. 2 is a diagram illustrating traditional transmission line repair.
Referring now to FIG. 2, an example of the transmission line repair when the optical fiber transmission line 3 in a relay section between two optical amplifiers 4 being cut off is shown. A transmission system of this type of configuration is used, for example, in an optical submarine cable system.
After removing the portion of the cable taking in water and connecting up both ends of the cut cable, a new optical submarine cable is inserted and is longer than the originally installed cable due to the connection being made on board a ship. The length of the new cable is determined by the depth of the water and must be at least twice the depth of water. The inserted cable is a fiber cable which is similar to the cable used as the optical fiber transmission line 3. There may be times when the many relay sections, created where the optical amplifiers split the cable between the optical send and the optical receive stations, use several different types of fiber. In this case, the inserted cable for repair in the transmission line uses a plurality of transmission lines having different dispersion values, respectively. The lengths of the transmission lines having the different dispersion values are adjusted and the overall dispersion value of the inserted cable is essentially zero. This allows for a reduction in misalignment caused by accumulated dispersion since the time the transmission system was originally installed.
Assume, for example, a non-zero dispersion shifted fiber (NZ-DSF) has a chromatic dispersion of −2 ps/nm/km and nine transmission sections are used. In addition, a 1.3 μm zero dispersion fiber (Single Mode Fiber (SMF)) has an optical dispersion of +18 ps/nm/km and one transmission section is used. The ratio of NZ-DSF and SMF would be adjusted to 9:1. In this way, cables having a dispersion value of essentially zero could be inserted which would allow for a reduction in the accumulated dispersion that accompany the inserted cables when the WDM optical transmission system is first installed.
Optical submarine cable systems are broken into the following three principal groups: shoreline, shallow ocean, and deep sea.
FIG. 3 is a diagram illustrating the types of sections of a submarine cable.
Referring now to FIG. 3, the shoreline section is the relay section that runs from the optical send station 1 to a first optical amplifier 4-1. Also, the shoreline section runs from a first optical amplifier 4-17 to the optical receive station 2. The shallow ocean section is the relay section in which optical amplifiers 4-2 through 4-6 are arranged in water up to approximately 1000 m deep. The deep sea section is the relay section is arranged from optical amplifiers 4-7 through 4-15 in water approximately 1000 m deep or deeper.
The frequency of repairs in each of the respective sections varies. Also, the deeper the water, the less frequent the repairs and a longer repair cable is needed.
Traditionally, the fiber used in shallow ocean, deep sea, and shoreline sections were of the same type. The submarine cable used in the respective sections were of different types of cables (i.e., the shoreline section of submarine cable was stronger than the shallow section of submarine cable, which was stronger than deep section of the submarine cable).
In a long distance, large capacity wavelength division multiplexing transmission system, the use of fiber having a large average mode field diameter and a small difference in accumulated chromatic dispersion between wavelengths is advantageous due to the reduced non-linear effect.
The transmission line uses a positive dispersion fiber (+D fiber) having a positive dispersion value in response to optical signal wavelengths transmitted over the first half of the relay section transmission line where the mode field diameter is large. Also, the transmission line uses a negative dispersion fiber (−D fiber) having a negative dispersion value in response to optical signal wavelengths where the chromatic dispersion and the chromatic dispersion slope from the first half of the fiber are compensated and the mode field diameter in the second half of the transmission line is small.
FIG. 4 is a diagram illustrating a typical configuration of a wavelength division multiplexing transmission system using positive dispersion and negative dispersion.
Referring now to FIG. 4, the WDM transmission optical system is made up of, for example, the optical send station (OS) 1, the optical receive station (OR) 2, the optical fiber transmission line 3 for connecting the optical sending and receiving stations, and several optical amplifiers 4 placed at the required intervals along the optical fiber transmission line 3.
The optical send station 1 and the optical receive station 2 are configured in the same way as in FIG. 1.
The optical fiber transmission line 3 contains several relay sections connecting the optical send station 1, each of the optical amplifiers 4, and the optical receive station 2. Each of the relay sections uses a hybrid transmission line configured as follows: The first half (sending side) uses the 1.3 μm zero dispersion SMF 3a, having a positive chromatic dispersion value and a positive dispersion slope, in the wavelength band of the WDM signal light. The second half uses the dispersion compensating fiber 3b having a negative chromatic dispersion value and a negative dispersion slope in the wavelength band of the WDM signal light.
In this example, the average chromatic dispersion in the sections of the hybrid transmission line made up of the positive dispersion fiber 3a and the negative dispersion fiber 3b, are set up as approximately −2 ps/nm/km. The accumulated chromatic dispersion is compensated by installing section (a) and (b) of FIG. 4 with only the positive dispersion fiber 3a. 
FIG. 5 is a diagram illustrating a typical dispersion map of a wavelength division multiplexing transmission system.
Referring now to FIG. 5, the characteristics in the diagram are shown for the multiplexing of 34-wave optical signals with a optical signal channel interval of 50 GHz. In the diagram, NZ-DSF is used as the transmission line. A triangle represents channel 34, a black square represents channel 17, and a circle represents channel 1. Characteristics without a mark used the positive dispersion and negative dispersion fiber as the transmission line as in FIG. 4. Transmission lines that used positive dispersion and negative dispersion fiber had a smaller average dispersion slope. Thus, their accumulated dispersion following transmission was smaller and transmission waveform distortion was reduced.
The following problems are encountered when repairing optical lines with +D/−D fiber.
(1) Transmission Waveform Distortion Caused by Dispersion Management and the Accumulative Dispersion Amount.
Table 1 shows the chromatic dispersion values of the conventional transmission line NZ-DSF as well as those of positive dispersion fiber and negative dispersion fiber.
TABLE 1Transmission Line Fiber Chromatic Dispersion ValueNegativePositive DispersionDispersionNZ DSFFiberFiberFiber(Traditional)ChromaticApproximately +20ApproximatelyApproximately −2Dispersion−20~−90Amount(ps/nm/km)
The absolute value of the chromatic dispersion amount per unit of length of positive dispersion and negative dispersion fiber are both very significant and are more than the traditional transmission line NZ-DSF by a factor of 1. For this reason, when making transmission line repairs using transmission lines made up of systems like the traditional ones in transmission systems using positive dispersion and negative dispersion fiber, a great deal is lost in the dispersion management of such systems and the accumulative dispersion amount following transmission changes significantly. This causes a large increase in waveform distortion and the transmission characteristics degrade.
(2) Increases Connection Losses
Table 2 shows the mode field diameter of positive dispersion fiber, negative dispersion fiber, NZ-DSF, and SMF.
TABLE 2Mode Field Diameter for Each Transmission Line FiberPositiveNegativeDispersionDispersionFiberFiberFiberNZ-DSFSMFModeApproximatelyApproximatelyApproximatelyApprox-Field10~124~67~8imatelyDiameter10~12(μm2)
The difference between the mode field diameters of the positive dispersion and negative dispersion fiber is larger than the differences between the mode field diameters of the SMF and NZ-DSF used in traditional transmission systems. Thus, the connection loss of the transmission line fiber is also larger. This effect becomes marked when the frequency of repairs is high or when the relay section is short.
(3) Excessive Increase in the Non-Linear Effect in the Negative Dispersion Fiber
When reducing the chromatic dispersion effect using a combination of positive dispersion and negative dispersion fiber and when trouble occurs in the first half of the relay section, the effect of the non-linear effect that occurs in the negative dispersion fiber making up the inserted cable causes the waveform distortion to have a considerable effect on the transmission characteristics. Therefore, light power of sending signals are high power.